1. Field of the Invention
The invention relates to a charging device for an image forming apparatus, which charges an image carrier such as a photo-sensitive drum or pape, and more particularly to a charging roller contacting to the image carrier.
2. Description of the Related Art
In electrostatic recording or electrophotographic recording, it is known that a corona charger is used for charging a photo-sensitive drum or for transferring an image formed on the photo-sensitive drum to a recording paper. In recent years, since the problems have risen that corona charging produces ozone which is harmful to human bodies and that actions for safety must be taken because the corona charger needs a high voltage power supply, a charging apparatus without using the corona charger is desired. By way of an example of charging without the corona charger, Japanese Patent Disclosure (kokai) No. 210862/88 discloses a brush charging roller and Japanese Patent Disclosure No. 267667/89 (kokai) discloses a contact charging method for charging a photo-sensitive drum with a biased charging roller contacting to the drum surface.
However, since, this contact charging method tends to make the drum surface non-uniformly charged, fogging or non-uniform shading would appear on developed images. A reason why the drum surface is not uniformly charged is as pointed out, because the resistance of the charging roller is not appropriate. Although a typical electronic photo-copying apparatus comprises a cleaner for cleaning the surface of the photo-sensitive drum after developing, toner particles, paper, and fine particles such as silica contained in the toner (collectively referred to as toner particles) are not completely removed and left on the drum surface, whereby these particles may adhere to the charging roller to cause so-called filming phenomena that the surface of the roller is covered with the particles. Owing to the filming phenomena, the surface resistance of the roller is changed. The filming phenomena is remarkable in a charging roller of a solid roller type. On the other hand, in the case of a charging roller of a brush roller type, although there are advantages that the brush roller is inexpensive compared with the solid roller and high accuracy is not required during mounting, non-uniform charging would happen owing to the construction of and the method of fabricating the brush, and lines or marks would appear in half tone images.